1. Field of the Invention
The present invention relates generally to a configuration of electrodes in a high voltage interrupter. More particularly, the present invention relates to electrodes specifically designed to eliminate arc stalling in a high voltage vacuum interrupter.
2. Summary of the Prior Art
The present invention is used in high voltage interrupters. An interrupter is a device containing two electrode heads through which high current is transferred as the electrodes are in closed position. When the overload current reaches a certain predetermined level the electrodes separate preventing further current flow across the electrodes. The interrupter works much like a fuse. When a large current flows across a fuse it "burns" out to prevent overload current flow. The difference between an interrupter and a fuse is that once a fuse is blown it has to be replaced. Contrarily, an interrupter's electrode are placed back in contact with one another once the overload current has subsided, thus it can be used continuously.
When the electrodes are separated, because of overload current, at any time during the sinusoidal period of the AC current that is not a zero point, an arc occurs between the separated electrodes until the next zero point is reached at which the arc is to be completely extinguished as this current zero point.
This arc can be damaging to the electrode contact faces. First, an arc current can ionize the electrode material causing the electrode surfaces to pit. This pitting causes an uneven contact face which results in surface roughness on the contact faces. Second, arcing can heat up the surface of the contact faces to their melting point. When this occurs the electrodes may fuse together when they recombine.
Conventionally, the vacuum interrupter is characterized by its rapid recovery of the dielectric strength of the vacuum gap at the critical current zero point. The presence of arc vapor from arcing affects the ability of the contact gap to withstand high voltage stress. The recovery characteristics are influenced not only by the electrode material but also by the electrode geometry.
It is well known that at high current level, the arc column is constricted under the influence of the local magnetic field which aggravates the local heating of the electrode and hence the metal vapor dispersed into the vacuum gap accelerates. It is also well known that the local magnetic fields from the current passing through the electrodes can act on the current forming the arc to cause the latter to move across the electrode. One effective method of avoiding local overheating of the electrode surfaces is to ensure that the arc moves over the surface by the self-induced magnetic field interacting with the arc column in order to spread out the local heat concentration.
Different types and shapes of electrode geometries have been studied to achieve continuous movement of the arc by the self-induced field. The first of these is discussed in FIG. 4. The second is discussed in FIG. 5.
Referring to FIG. 4, a top view of an electrode 8 is shown. The region from circular line 16 to circular line 18 is the contact face 17 of the electrode. It is raised vertically above (out of the paper) from an inner portion 19, inside line 18, and flanges 10-15 which extend outside of line 16. The inner portion 19 is depressed initiating the self induced magnetic field to move the arc from the contact face 17 out onto the flanges 10-15. The geometry of the electrode 8 is configured so that the arc will run out on one of the flanges 10-15. The reason why the arc moves is described in more detail below in the detailed description.
The problem with the configuration of FIG. 4 is that the arc stalls on the isolated flange since the flanges are significantly isolated from one another. This means it runs from the contact face 17 of the electrode to a particular flange and stays on that particular flange due to inefficient arc rotation of such isolated flange. Eventually the isolated flange will melt away or into another flange destroying its ability to maintain the arc away from the contact face. Over time all of the flanges are reduced by heating and pitting. The term used to describe when an arc slows down or stops on a particular segment of the electrode causing that segment to heat up is called arc stalling. Eventually the contact faces bare the local heating of the arc which increases the metal vapor, resulting in inability of dielectric recovery.
To overcome stalling on a particular flange a configuration has arose which allows the arc to jump from one flange to another, eliminating stalling on one flange. This configuration is shown in FIG. 5. The center portion 22 is cut below the contact face 23. Outside the contact face is a periphery plate 24 sloped away from the contact face 23. Rectangular slots are cut through the contact face 23 and the periphery plate 24 to create a plurality of flanges 26-29. The slots separating the flanges 26-29 promote directional self-induced magnetic field. Their geometry is intended to permit the arc to rotate from one flange the other. By permitting the arc to jump from one flange to another this configuration reduces arc stalling, which in turn adds to the longevity of the flanges 26-29 and the contact face 24.
The problem with the configuration of FIG. 5 is that the slots are cut rectangularly. The magnetic field created in the contact face which forces the arc to wave outward has to push the arc through two right angles. Focusing on flange 26, a magnetic field is created by the electrical current. The magnetic field is perpendicular to the x-axis and pushes the arc outward from the center along the x-axis. Within a very short distance, however, the flange 26 makes a right angle. The arc has developed little velocity by this point and the magnetic field is weak. Thus, the arc undergoes stalling as it makes the turn. This problem is compounded by a second right angle on the flange after it has left the contact face 23 and entered the periphery plate 24. There the arc undergoes similar stalling.
The slowing down of the arc at the two right angles increases the time it takes the arc to move off of the contact surface 23. That increases the damage to the contact surface 23 caused by pitting and overheating. The pitting and overheating caused by the arc stalling at the right angles will allow the electrodes to fuse when recombined.
Another shortcoming of the device of FIG. 5 is that it is made quite large. The rationale behind its size is that the larger the periphery, the further the arc will be from the contact faces, i.e., the less heat build up on the contact faces and the less pitting. Unfortunately, the larger the electrode the larger its housing has to be, thus the less economical of the design of the electrode.
Thus Applicant has found it not only desirable to eliminate arc stalling on the electrode but also desirable to reduce the size of the electrode for a given power rating.